overlay with 2 imagefields

With the underlay and overlay effects you cannot refer to another imagefield. However, you can refer to other images on your website using either one of the valid schemes, an absolute path, or a relative path (relative to the current directory, probably the Drupal site root). Valid schemes are a.o. private://, public://, and if the System Stream Wrapper module has been installed, also module:// and theme://.

If however you want to combine two dynamic images simultaneously attached to the same node, then knowing some absolute URL schemes may not help.

I can IMAGINE doing it with a customaction, assuming that we can deduce and load the container entity, then you could find the OTHER image also attached to this thing and overlay it or something.
You'd have to go down to PHP customaction for that though, so you can describe and do what you really want.

I can't imagine a UI for managing multiple layers (tried to build one in the very first iteration many years ago - it could be done but I failed to make the terminology make sense.)

D6 EOL.